In the early prior art of continuous casting utilizing one or more tensed endless metallic belts, the commercial casting of slab or thin metal strip sometimes had to be interrupted because of poor surface of the cast product or uneven product thickness or both. Such interruptions were especially likely to occur when certain difficult metals or alloys were being cast. Several advances in methods and apparatus evolved in more recent prior art and contributed to improvements in surface characteristics and uniformity of thickness in products being cast. Some of these improvements can become optimally effective only if continual ongoing information is immediately obtained during casting relating to the state of the casting belts and their insulating coatings. Such continual ongoing immediate information has heretofore not been reliably obtainable.
A major proximate cause of defective cast metallurgy or surface flaws has been the inability of a metallic casting belt to maintain continual and continuous contact with the freezing product. Sometimes non-flatness inheres in a new casting belt. Sometimes non-flatness is due to the distortions of the belt under the thermal effects of molten metal becoming solidified. Either way, non-flatness, even a relatively small amount of non-flatness can interrupt uniform heat extraction. In consequence, zones of nearly frozen alloy may suck late-freezing constituents from less-frozen zones that have lost contact with a casting belt, resulting in a totally unacceptable metallurgical structure.
Continuously moving casting belts are naturally subjected to great and varying thermally and mechanically induced stresses as the result of their exposure on one side to freezing molten metal, while on the other side being exposed to fast-flowing cooling water. At the same time, the belts in contact with the solidifying metal must lie flat and be steered or adjusted intermittently in order to conform approximately to true endless paths around which they are desired to be revolved. The heating of one surface of a metallic casting belt by molten metal naturally tends to expand that surface, causing compressive stress on that side. Because the other side of the belt near the fast-flowing liquid coolant remains relatively cold, the heating tends to distort the belt (in the area where it follows a nominally straight course), with the hot side tending to become convex. If the heating is non-uniform, as often occurs, flutes and ripples can be caused in the belt, and these distortions disturb the belt's contact with the freezing metal product, with the unwanted results mentioned above. Approximate flatness of the course of a belt is nevertheless maintained by exerting high tension on it, but tension alone may not be a sufficient mechanical control to prevent induced distortions in the casting of some metals.
The casting belts which are employed for linear belt-type casting, as in twin-belt casting, may be made for example of mild cold-finished steel or of copper alloy as described in U.S. Pat. No. 4,915,158 of J. F. Barry Wood, which is assigned to the same assignee as the present invention. The belt thickness typically lies between 0.035 and 0.065 of an inch (0.9 mm to 1.7 mm), though the thickness may lie somewhat outside this range.
For casting slab, the belts must be relatively wide. They normally first undergo a process of roller-stretch leveling as described in C. W. Hazelett's U.S Pat. No. 2,904,860, or they are mechanically prestrained in zones as in U.S. Pat. No. 4,921,037 of N. J. Bergeron, J. F. B. Wood, and R. W. Hazelett, assigned to the same assignee as the present invention. Such pre-treatments result in an extremely flat or well-proportioned belt, suitable for all current twin-belt continuous casting purposes. However, the thinness, the long and wide dimensions, weight, and moderate yield point of such relatively wide casting belts all add up to relative fragility, such that the belt, in its ordinary handling involved in crating, shipping, and mounting on a casting machine, may yield locally and so develop subtle undulations ("loops" or "nodes") which, though they may be difficult to see, impair usefulness in service despite the usual exertion of high tension during casting, which tends to keep belts flat. It is important for a casting operator to learn of such subtle belt imperfections before attempting to cast and during casting, so that the operator can correct the situation.
The employment of thermally insulative coatings on the outside (casting side) of such belts, i.e., on the side next to the freezing metal, has proved necessary for maintaining belt flatness and desired belt surface characteristics and effects during casting and hence for maintaining high qualities in cast products. These coatings on metallic casting belts control the belt temperatures resulting from contact with molten metal on the hot side of the belt. Both solid and liquid coatings have been used, often in combination. They will be described in detail later.
Degradation of the cast product is likely to occur when the insulative coating or coatings become thin or worn, or conversely when an uneven build-up occurs in a continually applied coating.
It would seem easy to install a mechanical, directly-contacting device to sense and indicate variations in the flatness of belts as they revolve around their respective carriages in a twin-belt casting machine and travel past such a directly-contacting device. A directly-contacting device is disclosed in U.S. Pat. No. 4,002,197, assigned to the same assignee as the present invention. But in fact, wear, vibration, and sticking have prevented such directly-contacting devices from being as practical in various continuous casting installations during day-after-day operations. Through prolonged exposure to fast-moving coolant, directly-contacting devices accumulate dirt, oil, and minerals. Moreover, the high levels of sensitivity that have recently proven to be desirable for ensuring optimum casting have unexpectedly rendered contact-type mechanical devices relatively marginal in their performance. Further, there has been difficulty of access to such directly-contacting devices for providing maintenance to them, because they were located among numerous closely-spaced backup rollers, nozzles, and gutters.
The present invention solves, or substantially overcomes, these problems of the prior art.